Comparison of 2 and 4 Pole Generators in Aerospace Applications

Traditionally aerospace applications have used 400hz 115/200VAC electrical power. While new commercial aircraft are now using variable frequency power – to eliminate the need for a hydromechanical constant speed drive or electronic frequency conversion, there are still may applications for 400Hz power – such as auxiliary power unit generators. For these applications either a 2 pole or 4-pole configuration generator could meet the requirement. For a given output frequency, such as 400 Hz, a 2 pole generator will spin twice as fast as a 4 pole generator does. Based on the generalized machine principles, it might be expected that a 4 pole generator be more or less 2 times as heavy as its 2 pole counterpart. During our trade studies it has been found that this is not necessarily the case. Many other factors like cooling schemes, rotor critical speed and mechanical stress considerations play an important role and can drive the design. In some cases, a 2 pole design may not be too much different in its weight compared with a 4 pole generator. This paper will present discussion on different considerations regarding 2 pole and 4 pole aerospace generator applications.